[fix][broker] In replication scenario, remote consumer could not be registered if there has no message was sent (apache#20888)

Motivation: In the replication scenario, we want to produce messages on the native cluster and consume messages on the remote cluster, the producer and consumer both use a same schema, but the consumer cannot be registered if there has no messages in the topic yet.The root cause is that for the remote cluster, there is a producer who has been registered with `AUTO_PRODUCE_BYTES` schema, so there is no schema to check the compatibility.

Modifications: If there is no schema and only the replicator producer was registered, skip the compatibility check.

Author: poorbarcode

pulsar-broker/src/main/java/org/apache/pulsar/broker/service/AbstractTopic.java

@@ -139,7 +139,7 @@ public abstract class AbstractTopic implements Topic, TopicPolicyListener<TopicP
 
     private static final AtomicIntegerFieldUpdater<AbstractTopic> USER_CREATED_PRODUCER_COUNTER_UPDATER =
             AtomicIntegerFieldUpdater.newUpdater(AbstractTopic.class, "userCreatedProducerCount");
-    private volatile int userCreatedProducerCount = 0;
+    protected volatile int userCreatedProducerCount = 0;
 
     protected volatile Optional<Long> topicEpoch = Optional.empty();
     private volatile boolean hasExclusiveProducer;

pulsar-broker/src/main/java/org/apache/pulsar/broker/service/persistent/PersistentTopic.java

@@ -3272,7 +3272,7 @@ public CompletableFuture<Void> addSchemaIfIdleOrCheckCompatible(SchemaData schem
                             .toList().size())
                     .sum();
             if (hasSchema
-                    || (!producers.isEmpty())
+                    || (userCreatedProducerCount > 0)
                     || (numActiveConsumersWithoutAutoSchema != 0)
                     || (ledger.getTotalSize() != 0)) {
                 return checkSchemaCompatibleForConsumer(schema);

pulsar-broker/src/test/java/org/apache/pulsar/broker/service/OneWayReplicatorTest.java

@@ -19,13 +19,18 @@
 package org.apache.pulsar.broker.service;
 
 import static org.testng.Assert.assertEquals;
+import static org.testng.Assert.assertFalse;
 import static org.testng.Assert.assertTrue;
+import java.util.Optional;
 import java.util.concurrent.TimeUnit;
 import org.apache.pulsar.broker.BrokerTestUtil;
+import org.apache.pulsar.broker.service.persistent.PersistentTopic;
 import org.apache.pulsar.client.api.Consumer;
 import org.apache.pulsar.client.api.MessageId;
 import org.apache.pulsar.client.api.Producer;
+import org.apache.pulsar.client.api.Schema;
 import org.apache.pulsar.common.policies.data.TopicStats;
+import org.awaitility.Awaitility;
 import org.testng.annotations.AfterClass;
 import org.testng.annotations.BeforeClass;
 import org.testng.annotations.Test;
@@ -75,4 +80,29 @@ public void testReplicatorProducerStatInTopic() throws Exception {
             admin2.topics().delete(topicName);
         });
     }
+
+    @Test
+    public void testCreateRemoteConsumerFirst() throws Exception {
+        final String topicName = BrokerTestUtil.newUniqueName("persistent://" + defaultNamespace + "/tp_");
+        Producer<String> producer1 = client1.newProducer(Schema.STRING).topic(topicName).create();
+        // Wait for replicator started.
+        Awaitility.await().untilAsserted(() -> {
+            Optional<Topic> topicOptional2 = pulsar2.getBrokerService().getTopic(topicName, false).get();
+            assertTrue(topicOptional2.isPresent());
+            PersistentTopic persistentTopic2 = (PersistentTopic) topicOptional2.get();
+            assertFalse(persistentTopic2.getProducers().isEmpty());
+        });
+        // The topic in cluster2 has a replicator created producer(schema Auto_Produce), but does not have any schema。
+        // Verify: the consumer of this cluster2 can create successfully.
+        Consumer<String> consumer2 = client2.newConsumer(Schema.STRING).topic(topicName).subscriptionName("s1")
+                .subscribe();;
+
+        // cleanup.
+        producer1.close();
+        consumer2.close();
+        cleanupTopics(() -> {
+            admin1.topics().delete(topicName);
+            admin2.topics().delete(topicName);
+        });
+    }
 }

pulsar-broker/src/test/java/org/apache/pulsar/schema/SchemaTest.java

@@ -1290,6 +1290,33 @@ private void testIncompatibleSchema() throws Exception {
         assertThrows(SchemaSerializationException.class, message2::getValue);
     }
 
+    /**
+     * This test just ensure that schema check still keeps the original logic: if there has any producer, but no schema
+     * was registered, the new consumer could not register new schema.
+     * TODO: I think this design should be improved: if a producer used "AUTO_PRODUCE_BYTES" schema, we should allow
+     *       the new consumer to register new schema. But before we can solve this problem, we need to modify
+     *       "CmdProducer" to let the Broker know that the Producer uses a schema of type "AUTO_PRODUCE_BYTES".
+     */
+    @Test
+    public void testAutoProduceAndSpecifiedConsumer() throws Exception {
+        final String namespace = PUBLIC_TENANT + "/ns_" + randomName(16);
+        admin.namespaces().createNamespace(namespace, Sets.newHashSet(CLUSTER_NAME));
+        final String topicName = "persistent://" + namespace + "/tp_" + randomName(16);
+        admin.topics().createNonPartitionedTopic(topicName);
+
+        Producer producer = pulsarClient.newProducer(Schema.AUTO_PRODUCE_BYTES()).topic(topicName).create();
+        try {
+            pulsarClient.newConsumer(Schema.STRING).topic(topicName).subscriptionName("sub1").subscribe();
+            fail("Should throw ex: Topic does not have schema to check");
+        } catch (Exception ex){
+            assertTrue(ex.getMessage().contains("Topic does not have schema to check"));
+        }
+
+        // Cleanup.
+        producer.close();
+        admin.topics().delete(topicName);
+    }
+
     @Test
     public void testCreateSchemaInParallel() throws Exception {
         final String namespace = "test-namespace-" + randomName(16);